Chunks near spawn do not unload.
Unoccupied overworlds do not unload.

Simply put: If there is no one on the server, then the server should be spending it's time updating zero blocks, zero entities, zero anything, and just waiting for either: 1, a connection on the socket for someone checking in, or 2, every 1/20th of a second seeing that it has nothing to do.

More to the point, if there are two of us on the server, off in the distance, then the only area of the map that should be loaded / consuming memory and CPU are the blocks near us, not anything near spawn where we haven't been in half an hour.

Similar issues exist with the nether being loaded even if no one is there (message logged about the nether on server startup), or the overworld still being loaded even if everyone is in the nether.

Please note: I constantly see 4 region files updated every minute that the server is up and running, even when no one is on. These are the region files containing the chunks around spawn. This means that every hour my backup is making a duplicate of "changed" files that have no new or altered information.